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Summary

Summary

The pilot reported that he was making an adjustment to the fuel selector, at the top of climb, when he noticed a lower than expected reading on the right outboard fuel quantity gauge. Suspecting a loose fuel cap, he advised the co-pilot to carry out a visual inspection of the right wing area with a torch. Although the fuel cap appeared to be seated properly, the decision was made to return to the DEPARTURE runway and carry out a more thorough investigation. During the landing roll the right engine lost all power, and the pilot attempted several unsuccessful restarts. At the end of the runway the aircraft was turned tail into wind and another restart was attempted. As the engine started the co-pilot observed flames under the right wing area and activated the engine fire extinquisher. The co-pilot then exited the aircraft to investigate and returned immediately to inform the pilot that the aircraft was on fire. Both engines were shut down and the pilots exited the aircraft. The investigation showed that the rubber fuel cells inside the right wing had collapsed upward towards the fuel filler inlet. The fuel drain fitting located directly behind the right engine exhaust was missing. Rubber fuel cells are known to collapse in this manner when a fuel filler cap comes loose in flight. The fuel is then sucked out through the filler neck into the low pressure area above the wing. This occurs faster than the vent system can cope, causing the inner cell to collapse. In this case, the fuel caps were found properly secured. However, the rubber sealing ring on the right fuel cap was perished and cracked in several places. The steel mating ring around the filler neck, which accepts the fuel cap, was also badly corroded over the entire sealing surface area. It is probable that the fuel syphoning began as a result of the poor seal around the cap. The syphoning would have been assisted by the tanks having been filled to the top of the filler neck for the flight. (The tanks had not been filled to this level for at least two years.) The outboard fuel (water) drain fitting attaches to a section of the fuel cell which protrudes through the lower wing surface. This fitting is located immediately behind the right engine exhaust outlet. When the fuel cell collapsed inside the wing, the fitting was unable to pass through the wing cut-out and was pulled from its attachment point on the cell. This then allowed fuel to flow freely into the airstream directly behind the engine exhaust. When the engine start was attempted with the aircraft tail into wind, the stream of fuel was blown directly back against the exhaust outlet. The investigation confirmed the right outboard exhaust as the ignition source for the fire.

 
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